Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SELECTION OF POSITIONING HANDOVER
CANDIDATES BASED ON PATH LOSS
BACKGROUND OF THE PRESENT INVENTION
Field of the Invention
The present invention relates generally to telecommunications systems and
methods for determining the location of a mobile terminal within a wireless
network, and specifically to an improved method, system, and apparatus for
determining nodes which are the likeliest to yield the most accurate
triangulation
results.
Background and Objects of the Present ~nventiQn
Determining the geographical position of a mobile station within a wireless
network has recently become important for a wide range of applications. For
example, positioning services may be used by transport and taxi companies to
determine the location of their vehicles. In addition, for emergency calls, ~,
911
calls, the exact location of the mobile terminal may be extremely important to
the
outcome of the emergency situation. Furthermore, positioning services can be
used
to determine the location of a stolen car, for the detection of home zone
calls,
which are charged at a lower rate, for the detection of hot spots for micro
cells, or
for the subscriber to determine, for example, the nearest gas station,
restaurant, or
hospital.
Referring now to FIGURE 1 of the drawings, an exemplary wireless
network, such as a Global System for Mobile Communication (GSM) Public Land
Mobile Network (PLMN) 10, will be described. The PMLN 10 is composed of a
plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and an
integrated Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12,
in
turn, include a plurality of Location Areas (LA) 18, which are defined as that
part
of a given MSC/VLR area 12 in which a mobile station (MS) (terminal) 20 may
move freely without having to send update location information to the MSC/VLR
area 12 that controls the LA 18. Each Location Area 12 is divided into a
number of
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cells 22. Mobile Station (MS) 20 is the physical equipment, g~, a car phone or
other portable phone, used by mobile subscribers to communicate with the
cellular
network 10, each other, and users outside the subscribed network, both
wireline and
wireless.
The MSC 14 is in communication with at least one Base Station Controller
(BSC) 23, which, in turn, is in contact with at least one Base Transceiver
Station
(BTS) 24. The BTS is the physical equipment, illustrated for simplicity as a
radio
tower, that provides radio coverage to the geographical part of the cell 22
for which
it is responsible. It should be understood that the BSC 23 may be connected to
several base transceiver stations 24, and may be implemented as a stand-alone
node
or integrated with the MSC 14. In either event, the BSC 23 and BTS 24
components, as a whole, are generally referred to as a Base Station System
(BSS)
25.
With further reference to FIGURE l, the PLMN Service Area or wireless
network 10 includes a Home Location Register (HLR) 26, which is a database
maintaining all subscriber information, g,,g" user profiles, current location
information, International Mobile Subscriber Identity (IMSI) numbers, and
other
administrative information. The HLR 26 may be co-located with a given MSC 14,
integrated with the MSC 14, or alternatively can service multiple MSCs 14, the
latter of which is illustrated in FIGURE 1.
The VLR 16 is a database containing information about all of the Mobile
Stations 20 currently located within the MSCNLR area 12. If a MS 20 roams into
a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data
about that Mobile Station 20 from the HLR database 26 (simultaneously
informing
the HLR 26 about the current location of the MS 20). Accordingly, if the user
of
the MS 20 then wants to make a call, the local VLR 16 will have the requisite
identification information without having to reinterrogate the HLR 26. In the
aforedescribed manner, the VLR and HLR databases 16 and 26, respectively,
contain various subscriber information associated with a given MS 20.
Referring now to FIGURE 2, the operation of a wireless network 205
performing a positioning handover for determining the location of a given MS
200
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is described. Upon a network positioning request, the Base Station System
(BSS)
(220 and 240) serving the MS 200 generates positioning data, which is
delivered to
the Mobile Switching Center (MSC) 260. This positioning data is then forwarded
to a Positioning Center (PC) 270 for calculation of the geographical location
of the
MS 200. The location of the MS 200 can then be sent to the application 280
that
requested the positioning. Alternatively, the requesting application 280 could
be
located within the MS 200 itself. The Positioning Center 270 could also be
located
within the MSC 260.
In order to accurately determine the location of the MS 200, positioning
data from three separate Base Transceiver Stations (210, 220, and 230) is
required.
This positioning data for GSM systems includes a Timing Advance (TA) value,
which corresponds to the amount of time in advance that the MS 200 must send a
message in order for the BTS 220 to receive it in the time slot allocated to
that MS
200. When a message is sent from the MS 200 to the BTS 220, there is a
propagation delay, which depends on the distance between the MS 200 and the
BTS 220. TA values are expressed in bit periods, and can range from 0 to 63,
with
each bit period corresponding to approximately 550 meters between the MS 200
and the BTS 220. It should be understood, however, that any estimate of time,
distance, or angle can be used, instead of the TA value of GSM systems.
Once a TA value is determined for one BTS 220, the distance between the
MS 200 and that particular BTS 220 is known, but the actual location is not.
If, for
example, the TA value equals one, the MS 200 could be anywhere along a radius
of
550 meters. Two TA values from two BTSs, for example, BTSs 210 and 220,
provide two possible points that the MS 200 could be located (where the two
radiuses intersect). However, with three TA values from three BTSs, g,,g~,
BTSs
210, 220, and 230, the location of the MS 200 can be determined with a certain
degree of accuracy. For example using a triangulation algorithm, with
knowledge
of the three TA values and site location data associated with each BTS (210,
220,
and 230), the position of the mobile station 200 can be determined (with
certain
accuracy) by the Positioning Center 270.
Therefore, Timing Advance (TA) values are obtained from the original
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(serving) BTS 220 and two neighboring (target) BTSs (210 and 230). In order
for
each target BTS (210 and 230) to determine a TA value, a positioning handover
to
each of the BTSs (210 and 230) must occur. A positioning handover is similar
to
an ordinary asynchronous handover. The target BTS, e.g:, BTS 210,
distinguishes
the Positioning Handover from an ordinary handover by a new ACTIVATION
TYPE in the CHANNEL ACTIVATION message. Unlike an ordinary handover,
upon reception of a HANDOVER ACCESS message from the MS 200, the target
BTS 210 only calculates the TA value, and does not respond to the mobile
station
200, that is, no PHYSICAL INFORMATION is sent to the MS 200. Thus, the MS
200 will then return to the previous channel allocated by the original BTS 220
after
the time period defined by the MS's 200 internal counter expires, e.g:, 320
milliseconds.
If there are more than three BTSs (210, 220, and 230) within the range of
the MS 200, the serving BSC 240 will have to determine to which two BTSs 210
and 230 to perform a positioning handover (in order to obtain the TA values).
In
addition, if the serving BTS 220 does not support positioning, three target
BTSs
must be selected. At present, this selection process is typically performed by
the
BSC 240 compiling a mobile assisted handover list based on measurements
obtained by the MS 200 regarding the signal strength of the surrounding BTSs
(210, 220 and 230). The BSC 240 then selects the two or three BTSs (220 and
230) with the strongest signal strength to perform a positioning handover.
Unfortunately, the selected BTSs (210, 220, and 230) may not be the ideal
candidates for obtaining positioning data. For example, if the BTSs (210, 220,
and
230) selected for positioning handovers do not surround the mobile station 200
to
be positioned, the error in the location calculation will increase.
It is therefore an object of the invention to determine the target BTSs that
are the likeliest to yield the best triangulation results.
It is also an object of the invention to determine the target BTSs in a
manner that is computationally simple.
It is also an object of the invention to determine the target BTSs in a
manner that requires minimal modifications to existing cellular networks.
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SUMMARY OF THE INVENTION
The present invention is directed to a method for determining target nodes
that are the likeliest to yield the best triangulation results by determining
the
strength of signals received at the receiver from various nodes, determining
the
strength of the signals at the nodes, and calculating a path loss for each
node by
subtracting the strength of the signal at the receiver from the strength of
the signal
at the target node. The present invention is also directed to a
telecommunication
system for determining the location of a mobile station including Base
Transceiver
Stations for transmitting a signal to the mobile station, path loss
measurements for
measuring the path loss of each Base Transceiver Stations, and selection of at
least
two of the Base Transceiver Stations with the lowest path loss. The present
invention is also directed to an apparatus for selecting target Base
Transceiver
Stations with memory for the output power of Base Transceiver Stations, inputs
for
receiving a measurement report measuring the strength of signals received at a
receiver, and a way to calculate the path loss of each Base Transceiver
Station, and
for selecting at least two Base Transceiver Stations having the lowest path
loss.
More specifically, in one aspect, the invention provides a method for
selecting a
positioning handaver node, the method comprising the steps of receiving a
multiplicity of
signals from a corresponding multiplicity of target nodes adjacent to a
receiver,
determining, at said receiver, the strengths of said multiplicity of signals,
and calculating
respective path losses for each of said multiplicity of target nodes. For a
serving node
which supports positioning handover, the method includes calculating a timing
advance
value associated with said serving node, and selecting at least two of said
target nodes
which support positioning handover, said at least two target nodes having the
lowest path
loss calculated during said step of calculating. For other serving nodes, the
method
includes selecting at least three of said target nodes which support
positioning handover,
said at least three target nodes having the lowest path loss calculated during
said step of
calculating, and sending a message to said selected target nodes, wherein said
message
instructs said selected target nodes to calculate respective timing advance
values.
34
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In another aspect, the invention provides a telecommunication system for
determining the selection of positioning handover Base Transceiver Stations,
the
telecommunications system comprising a multiplicity of Base Transceiver
Stations, each
said Base Transceiver Station transmitting a respective signal received by a
mobile
station, and path loss measuring means for measuring the respective path
losses between
each of said Base Transceiver Stations and said mobile station. For a serving
Base
Transceiver Station which supports positioning handover, the system includes
calculating
means for calculating a timing advance value associated with said serving Base
Transceiver Station, and selecting means for selecting at least two of said
multiplicity of
to Base Transceiver Stations having the lowest measured path loss. For other
serving Base
Transceiver Stations, said selecting means further selects at least three of
said
multiplicity of Base Transceiver Stations which support positioning handover,
said
selected Base Transceiver Stations having the lowest measured path loss, and
the system
includes message means for sending a message to said selected Base Transceiver
Stations, wherein said message instructs said selected Base Transceiver
Stations to
calculate respective timing advance values.
In another aspect, the invention provides a selection device for selecting a
lowest
path loss of target Base Transceiver Station within a telecommunications
system, the
selection device comprising memory means for storing output power data
corresponding
2o to a multiplicity of target Base Transceiver Stations within said
telecommunications
system, determining means for determining the strength of respective signals
transmitted
from said multiplicity of Base Transceiver Stations, and calculating means for
determining the respective path losses of said multiplicity of Base
Transceiver Stations,
said path tosses comprising the respective differences of the output power of
each said
Base Transceiver Station and the strength of said signals transmitted
therefrom. For a
serving Base Transceiver Stations which supports positioning handover, the
device
includes calculating means for calculating a timing advance value associated
with said
serving Base Transceiver Station, and selecting means for selecting at least
two of said
multiplicity of Base Transceiver Stations which supports positioning handover,
said
3o selected Base Transceiver Stations having the lowest measured path loss.
For other
serving Base Transceiver Stations, said selecting means further selects at
least three of
said multiplicity of Base Transceiver Stations which support positioning
handover, said
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selected Base Transceiver Stations having the lowest measured path loss, and
the device
includes message means for sending a message to said selected Base Transceiver
Stations, wherein said message instructs said selected Base Transceiver
Stations to
calculate respective timing advance values.
BRIEF DESCRIPTION OF THE DRAWINGS
The disclosed inventions will be described with reference to the
accompanying drawings, which show important sample embodiments of the
invention and which are incorporated in the specification hereof by reference,
wherein:
FIGURE 1 is a block diagram of a conventional terrestrially-based wireless
telecommunications system;
FIGURE 2 illustrates a sample positioning handover in which positioning
1s data is acquired by a target base transceiver station and transmitted to a
serving
base station controller;
FIGURE 3 is a flow chart demonstrating steps in a sample embodiment of
the target base station determination of the present invention;
FIGURE 4 is an illustration of a mobile telecommunication system
2o embodying the present invention; and
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FIGURE 5 is an exemplary illustration of the path loss determination of
Base Transceiver Stations.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED
EXEMPLARY EMBODIMENTS
The numerous innovative teachings of the present application will be
described with particular reference to the presently preferred exemplary
embodiments. However, it should be understood that this class of embodiments
provides only a few examples of the many advantageous uses of the innovative
teachings herein. In general, statements made in the specification of the
present
application do not necessarily delimit any of the various claimed inventions.
Moreover, some statements may apply to some inventive features but not to
others.
With reference now to FIGURE 3 of the drawings, steps in a sample
process for determining the optimal base transceiver stations in order to
locate a
Mobile Station 200 within a Location Area 205 is illustrated. Initially, after
a
positioning request is received by the MSC 260 (step 300) from the Positioning
Center 270, the MSC 260 sends the positioning request to an originating
(serving)
Base Station Controller (BSC) 240 (step 305) if the Mobile Station 200 is in a
dedicated mode (in use). However, if the MS 200 is in an idle mode (not in
use),
the MSC 260 must page the MS 200 and setup a call to the MS before forwarding
the positioning request to the BSC. This call does not activate the ringing
tone on
the MS 200, and therefore, is not noticed by the MS 200.
The originating BSC 240 then determines which Base Transceiver Station
(BTS) 220 is currently serving the MS 200 (step 310), and obtains a Timing
Advance (TA) value (TA 1 ), or other positioning data, from this serving BTS
220
(step 315), if possible. Thereafter, TA values are obtained from two target
BTSs
(210 and 230) (step 350) by performing a positioning handover (step 335). If
the
serving BTS 220 does not support positioning, an additional target BTS (not
shown) must be selected. It should be noted that other positioning methods
based
on triangulation can be used instead of obtaining TA values, as discussed
herein.
In addition, positioning of the MS 200 can be performed using more than three
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BTSs (210, 220, and 230).
Referring now to FIGURE 4, which will be discussed in connection with
FIGURES 2, and 3, the process by which the BSC 240 selects the target BTSs 210
and 230 (step 320) is described. The MS 200, as in most digital wireless
systems,
utilizes a method for automatically adjusting its transmitting power based on
the
strength of the signal received by the serving BTS 220. According to the
procedures of Mobile Assisted Hand Over (MAHO), the MS 200 measures the
strength of the received signal from the serving BTS 220, as well as a number
of
BTSs 420a - 420f serving each of the six surrounding cells 430a - 430f,
respectively. These measurements are reported to the BSC 240 in the MS
measurement report on the Slow Assisted Control Channel (SACCH) about twice
every second.
Upon receiving the measurements in the MS report, the BSC 240
determines the path loss of the serving BTS 220 and each neighboring BTS 420a -
420f. Referring now to FIGURE 5, an exemplary illustration of the BSC 240
determining the path loss of each neighboring BTS 420a - 420f is described.
The
column referenced as "Received Signal Strength" is provided by the MS 200 in
the
MS measurement report to the BSC 240. The column referenced as "Output
Power" includes data known, stored, and configured by the BSC 240. Those
skilled in the art will recognize that the BSC 240 is responsible for
controlling the
output power of the BTSs 220, and 420a - 420f. The BSC 240 can, for example,
store data regarding the output power of each BTS 220, and 420a - 420f,
respectively, in a database 245. The column entitled "Path Loss" represents
the
path loss of each respective neighboring BTS 420a - 420f.
The BSC 240 determines the information in the column entitled "Path loss"
according to the following formula:
path loss(n) = output power(n) - received signal strength(n)
where: n = row number.
The BSC 240 then ranks each neighboring BTS 420a - 420f in terms of path loss,
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as illustrated in the column entitled "Rank by path loss." The neighboring BTS
420
which reports the lowest path loss is ranked highest.
The BSC 240 then selects at least two of the neighboring BTSs 420 (at least
three if the serving BTS 220 does not support positioning) with the lowest
path loss
as the aforedescribed target BTSs 210 and 230 of FIGURE 2. In the exemplary
case of FIGURE 5, the BSC 240 would select BTS 420d and BTS 420e, to be the
target BTSs 210 and 230.
With reference again to FIGURE 3, after step 320 the positioning handover
to one of the target BTSs 230 (step 322) is accomplished by the serving BSC
240
sending a new ACTIVATION TYPE in a CHANNEL ACTIVATION message to
the target BTS 230, which informs the target BTS 230 that a positioning
handover
needs to be performed (step 325). The target BTS 230 then acknowledges the
CHANNEL ACTIVATION message to the serving BSC 250 (step 330).
Thereafter, the BSC 240 sends a command to the MS 200 via the serving
BTS 220 (step 335) to transmit a HANDOVER ACCESS message to the target
BTS 230 (step 340). During the time that the MS 200 is waiting for a response
from the target BTS 230, g,~,, around 320 milliseconds, the target BTS 230
measures the Timing Advance value (access delay) (TA3) (step 345), using
access
bursts sent by the MS 200, and forwards this positioning data to the serving
BSC
240 (step 350). A positioning handover can then be performed to the other
target
BTS 210 in the same manner as stated hereinbefore. The TA value measured by
the target BTS 230 (TA3) is then transmitted by the serving BSC 240 to the MSC
260 (step 355), together with TA values (TA1 and TA2) obtained from the
serving
BTS 220 and other target BTSs 210.
Finally, the TA value acquired from the target BTS 230 (TA3), together
with other TA values (TA1 and TA2) are forwarded to the Positioning Center
(PC)
270 from the MSC 260 (step 360), where the location of the MS 200 is
determined
using the triangulation algorithm (step 365). The PC 270 then presents the
geographical position of the MS 200 to the requesting application (node) 280
(step
370) for further processing.
As will be recognized by those skilled in the art, the innovative concepts
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described in the present application can be modified and varied over a wide
range
of applications. For example, it should be noted that the aforedescribed
determination of optimal target Base Transceiver Stations can be implemented
in
any cellular system, and should not be limited to GSM systems. For example, in
other cellular systems, the Base Station Controller function (controlling
node) can
be implemented within the MSC (such as 260) itself.
In another embodiment, the selection of the Base Transceiver Stations could
be made by the mobile station. In such an embodiment, each Base Transceiver
Station would report to the mobile station the transmitting strength. The
mobile
station could then determine the path loss of each Base Transceiver Station
and
select the optimal Base Transceiver Stations.
It should also be understood that the present invention is not limited to the
selection of two base transceiver stations. For example, in one embodiment, a
first
base transceiver station can be selected according to path loss while any
number of
other base transceiver stations can be selected according to some other
criteria.
Accordingly, the scope of the present invention should not be limited to any
of the
specific exemplary teachings discussed, but is only limited by the following
claims.
